The commercial development of the sleeping bag has been one of opportunism and myth. While all sorts of insulating materials have been used in sleeping bags the development of the down bag brought advertising campaigns with resulting misconceptions concerning various properties and characteristics of down and the unfavorable impressions regarding sleeping bags.
Traditionally sleeping bags were made with double walls and filled with various types of insulating material. They were bags of the same width from top to bottom and manufactured without zippers, or other closing means so that the user had to crawl in from the top and work his way down into the bag for proper rest. This was more efficient than was realized at first, because there was no opportunity for cold air entry or interior heat to escape except at the top. Zippers and other closures were added for convenience and quick operation, but zippers particularly, due to their construction, produced cold seams which are a factor in reducing the efficiency of the sleeping bag. In Europe zippers are not used because of the cold seam. It is now generally conceded that an efficiently made sleeping bag provides a layer of dead air, i.e. insulation around ones body. The thicker the layer of insulation, all other things being equal, the greater the warmth of the bag. However, the layer of insulation does not produce warmth, it simply traps body heat. With the recognition of these basic circumstances sleeping bags were made narrower and mummy-like in form. In this manner two inches of uniform insulation on a wide bag of the old style is less efficient than a narrow or tapered bag with the same two inch thickness of insulation because there is less inside air space to be heated by the body trapped in the insulation.
As stated earlier, the problem of preventing cold air from entering, and heat escaping through the zipper, arose with the advent of the zipper closure. Prior to the double zipper as disclosed herein, there were numerous attempts to stop the cold air seams and the escape of heat by the use of down-filled draft flaps located on the inner side of the sleeping bag along the sewing line of the zipper. Sometimes the flap was on the upper side of the zipper, sometimes it was on the lower side, and even in some instances on both sides of the zipper. These flaps were flexible, non-stationary projections which when properly aligned were intended to cover the zipper connection on the inside of the sleeping bag. Unfortunately, because these flaps were easily collapsed, or tended to move away from the zipper connection, the protection was inconsistent and the performance unsatisfactory. In addition, because they were connected to a portion of the bag a cold air inlet or a heat juncture was always present.